Understand your last post. Point I was making was that not all would define casual as you just have. In Radisson's world, sports coat without tie qualifies for informal. Casual, you can leave off the coat.
According to traditional etiquette, "informal" does require a tie -- but your post really illustrates the importance of reading each cruise line's description of its terminology because many cruise lines do take latitude.
IIRC, though, my description of casual indicated that a sport coat is optional -- which means that a shirt with an open collar and a pair of slacks. without the coat, is fine.